This invention relates generally to gas turbines and, more particularly, to methods and systems for operating a gas turbine.
At least some known gas turbines are coupled to and supply power to an electric power grid to facilitate maintaining a desired grid frequency for grid operation. Grid instabilities may cause the grid frequency to change, which may cause, the gas turbine to either increase or decrease power production to maintain the desired grid frequency. For example, an increase in grid frequency may result in an under-frequency event wherein the gas turbine produces less power than is desired by the grid. As a result, in such an event, fuel flow to the gas turbine must be increased to enable the turbine to meet the increased power requirements of the grid. Alternatively, a decrease in grid frequency may result in an over-frequency event wherein the gas turbine produces more power than is required by the grid. As such, in such an event, fuel flow to the gas turbine must be decreased to prevent instability within the gas turbine. Current grid code requirements necessitate rapid changes in fuel flow because failure to rapidly respond to an over-frequency or under-frequency event may cause power outages including brownouts and/or blackouts.
At least some known gas turbines operate such that an increase or decrease in fuel flow to a combustor is accompanied by a corresponding increase or decrease in air flow to the combustor. However, increasing or decreasing both the fuel flow and the air flow simultaneously may result in a combustion excursion, wherein the combustor becomes unstable. To prevent combustion excursion, fuel flow adjustments are initiated prior to airflow adjustments. Specifically, in at least some known gas turbine engines, measured compressor pressure ratio (CPR) and measured gas turbine exhaust temperature (TTXM) values are used to facilitate controlling fuel and air flow in response to grid demands. Specifically, fuel flow is sensed and controlled by a fuel valve position, and air flow is sensed and controlled by a compressor inlet guide vane position. Moreover, the CPR and TTXM are also used to define a state of the combustion system by controlling a fuel split to the combustor nozzles. The fuel split is sensed and controlled by a valve positioned in each of the combustor fuel legs. As such, a change in fuel flow demanded and/or air flow demanded will not result in a change to combustor fuel splits until the fuel flow/air flow changes produce a change in CPR or TTXM. As such, by design, the combustor state lags behind the state of the gas turbine engine.
Because of combustor state lags, large grid fluctuations, which generally cause rapid changes in fuel flow/air flow, may result in abnormal combustor operations. Specifically, during operations in which fuel flow is being decreased, the combustor may be susceptible to lean fuel blowout. Moreover, operating with lean fuel conditions may change dynamic pressure oscillations within the combustor, resulting in combustor instability. During operations in which fuel flow is being increased, combustor dynamic pressure oscillations may be generated which may cause combustor instability. Ultimately, combustor instability and/or a flame out may result in loss of power to the electric power grid.